RPLC-ESI-HRMS analysis of medium-chain dioic fatty acids in samples of aged siccative oil paintings

To tackle the ageing phenomena of precious artworks, the binding media have been increasingly explored in cultural heritage. When siccative oils are employed as paint media, medium-chain dicarboxylic fatty acids (DFAs), also known as dioic acids, are generated from unsaturated fatty acyl chains by hydroxyl and hydroperoxyl radicals during the drying process. Hitherto, gas-chromatography coupled with mass spectrometry (MS) has represented the foremost technique for the investigation of DFAs upon proper derivatization reactions, yet the current work aims to introduce an alternative approach based on reversed-phase liquid chromatography, negative-ion-mode electrospray ionization with high-resolution/accuracy MS and tandem MS. Painting reconstructions of linseed oil mixed with common inorganic pigments such as chrome yellow and cadmium red were investigated for their content of suberic (C8), azelaic (C9), sebacic (C10), and undecanedioic (C11) acids. For the identification of these DFAs as unbounded species, retention time data followed by confirmation of their accurate m/z measurements and a detailed analysis of their fragmentation patterns via tandem MS were used. To estimate the quantity of DFAs, samples were spiked with a suitable internal standard before lipid extraction. This analytical strategy was successfully applied to three 50-year-old oil paintings, using samples of only a few milligrams (i.e., 25–50 mg). Notably, a difference in DFA profiles was observed within the artwork, with yellow pigments exhibiting relatively lower content compared to red ones. Spectroscopic techniques revealed alterations relevant to the chemical composition of these pigments during the ageing process. Likely the chromate ion, a component of chrome yellow pigment, undergoes oxidation which may hinder the generation of hydroxyl and hydroperoxyl radicals by the Fenton reaction, as the iron catalyst naturally occurring in the drying oil is potentially affected.